Character analysis of the seed coat in Spiranthoideae and Orchidoideae, with special reference to the diurideae (ORCHIDACEAE)

Previous work on seed types within Orchidaceae has demonstrated that characters associated with the seed coat may have considerable phylogenetic utility. Application of these characters has been complicated in practice by the absence of quantitative descriptors and in some instances by their apparent lack of congruity with the taxa under consideration. Using quantitative descriptors of size and shape, we have demonstrated that some of the existing seed classes do not represent well delimited, discrete entities, and we have proposed new seed classes to meet these criteria. In the spiranthoid‐orchidoid complex, the characters yielding the most clearly delimited shape classes are cell number and variability and degree and stochasticity of medial cell elongation. Of lesser, but still appreciable, significance are the presence of varying types and degrees of intercellular gaps, and some, but not all, features of cell walls. Four seed classes are evident on the basis of these characters in Spiranthoideae and Orchidoideae. These seed types are briefly described, and their distribution among the taxa examined for this study is reported. It is hoped that these more strictly delimited seed classes will facilitate phylogenetic analysis in the family.